Legal psychology involves the application of the study and practice of psychology to legal institutions and people who come into contact with the law. Legal psychology is a field which takes basic social and cognitive theories and principles and applies them to issues in the legal system such as eyewitness memory, criminal and civil jury decision-making, investigations and interviewing. Most notably, legal psychologists have been involved in areas such as wrongful convictions and actual innocence cases, jury and trial consulting, as well as Department of Justice guidelines on eyewitness identification.

Legal psychology and forensic psychology together form the field more generally recognized as "psychology and law". Following up on earlier efforts by psychologists to address legal issues, psychology and law became a field of study in the 1960s as part of an effort to enhance justice. The American Psychological Association's Division 41, the American Psychology-Law Society, is active, and there are similar societies in Britain and Europe.

The field of psychology and law has been criticized for retreating from its initial critical focus on the law's role in maintaining injustice (see, e.g., article by Dennis Fox). While this criticism exists, it is rarely discussed and is basically irrelevant to the field as a whole. Many researchers do study injustice and similar issues, but as the field has grown researchers have determined ways to apply psychology specificially and empiricism more generally to legal issues. Many legal psychologists avoid the application of psychological "theory" and focus on demonstrating that legal questions can be answered through science.

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The fields of legal psychology are much like the fields of general psychology. For instance, cognitive psychologists may do legal research in eyewitness memory, eyewitness memory or perceptual issues involving crime. Social psychologists may study jury decision making, racial discrimination, etc. Psychologists can get involved in both criminal law and civil law research and application.

Psychologists specifically trained in legal issues, as well as those with no formal training, are often called by legal parties to testify as expert witnesses. In criminal trials, an expert witness may be called to testify about eyewitness memory, mistaken identity, competence to stand trial, the propensity of a death-qualified jury to also be "pro-guilt," etc. Psychologists who focus on clinical issues often testify specifically about a defendant's competence, intelligence, etc. More general testimony about perceptual issues (e.g., adequacy of police sirens) may also come up in trial.

Experts, particularly psychology experts, are often accused of being "hired guns" or "stating the obvious." Eyewitness memory experts, such as Elizabeth Loftus, are often discounted by judges and lawyers with no empirical training because their research utilizes undergraduate students and "unrealistic" scenarios. If both sides have psychological witnesses, jurors may have the daunting task of assessing difficult scientific information.

Legal psychologists may hold advisory roles in court systems. They may advise legal decision makers, particularly judges, on psychological findings pertaining to issues in a case. The psychologist who acts as a court advisor provides similar input to one acting as an expert witness, but acts out of the domain of an adversarial system.

Psychologists can provide an amicus brief to the court. The American Psychological Association has provided briefs concerning mental illness, retardation and other factors. The amicus brief usually contains and opinion backed by scientific citations and statistics. The impact of an amicus brief by a psychological association is questionable. For instance, Judge Blackmun once called a reliance on statistics "numerology" and discounted results of several empirical studies. Judges who have no formal scientific training also may critique experimental methods, and some feel that judges only cite an amicus brief when the brief supports the judge's predisposition.

Psychologists employed at public policy centers may attempt to influence legislative policy or may be called upon by state (or national) lawmakers to address some policy issue through empiricism and research. A psychologist working in public policy might suggest laws or help to evaluate a new legal practice (e.g., the use of sequential lineups v.s. simultaneous lineups).

Legal consultants, especially psychologists, are often hired to help pick juries, conduct mock trials, prepare witnesses, etc. Consultants may work for a law firm, be hired by one side in the case, or even be hired by a party in the case.

Most legal psychologists work for research institutions, such as universities, doing empirical research and teaching. These psychologists work in a variety of psychological areas while focusing some or all of their research on psycholegal issues. The published work of these researchers may be cited by policy makers or expert witnesses. Expert witnesses often do a great deal of research in the field to become "experts," and often utilize their own research results in forming an opinion in a particular case.

Research is also the best way for psychologeal scholars to get information to the public. Several journals publish research on psychology and the law, and newspapers often provide summaries of specific studies. Recently, a study concerning police lineups was the object of several news stories.

Many people who study some aspect of legal psychology have no particular legal training. Some may argue that specialized legal training actually dilutes the psychological empiricism of the researcher. For instance, to understand how eyewitness memory "works," a psychologist should be concerned with the whole picture instead of only aspects relevant to law (e.g., lineups, accuracy of testimony). Many psycholegal researchers focus on one aspect of psychology with no formal legal training and later apply that training to the law.

Today, several schools offer specialized training in legal psychology in some form. Some schools offer joint training in psychology and the law (through law schools) while others just focus on psycholegal issues. See US courses in psychology and the law

Many psychologists act as trial consultants. No special training nor certification is needed to be a trial consultant, though an advanced degree is generally welcomed by those who would hire the trial consultant. The American Society of Trial Consultants does have a code of ethics for members, but there are no legally binding ethical rules for consultants.

Some psychologists who work in academics are hired as trial consultants when their expertise can be useful to a particular case. Other psychologist/consultants work for or with established trial consultant firms. The practice of law firms hiring "in-house" trial consultants is becoming more popular, but these consultants usually can also be used by the firms as practicing attorneys.

The practice of a trial consultant being both a lawyer and a psychologists has its pros and cons. A legally trained psychologist is able to understand the issues of law and can also work for a firm as a practicing attorney. However, when a consultant is hired for psychological expertise, lawyers usually do not expect nor want the consultant to provide a legal opinion. The [[University of Nebraska-Lincoln] offers a master's of legal studies, which gives potential consultants training in law without a formal legal degree.

Trial consultants perform a variety of services for lawyers, such as picking jurors (usually relying on in-house or published statistical studies) or performing "mock trials" with focus groups. Trial consultants work on all stages of a case from helping to organize testimony, preparing witnesses to testify, picking juries, and even arranging "shadow jurors" to watch the trial unfold and provide input on the trial. There is some debate on whether the work of a trial consultant is protected under attorney-client privilege, especially when the consultant is hired by a party in the case and not by an attorney.

Those trained in legal psychology may be hired by psychology departments, criminal justice departments or even law schools to teach and conduct research. Like other professors, legal psychologists generally teach one or more classes a semester, mentor undergraduates or graduate students, and conduct research in a particular domain. Many legal psychologists conduct research in both legal issues, as well as in a more general area of psychology (e.g., memory or decision making) without a specific legal focus.

Jobs in academics require an advanced degree, and law schools usually require a formal law degree. Job competition in research-oriented, larger univerisities is fierce, especially since there are very few formal legal psychology programs to hire researchers. Those who are willing to work at smaller universities without a formal legal psychology program have more job opportunities.

Some legal psychologists who also have formal law degrees work as practicing attorneys. In this case, the legal psychologist may never rely on the psycholegal training, and the value of an advanced psychology degree for a practicing attorney is questionable.